As shown in FIG. 5, in a natural gas engine (CNG engine) 10X according to a conventional technology, natural gas fuel C compressed by a piston 63 is ignited by spark ignition using an ignition plug 62 provided in a cylinder head 61, and the natural gas fuel C is combusted.
The natural gas fuel C is injected into intake air A by a natural gas fuel jet injector (natural gas fuel injection device) 35 provided in an intake system passage 65 communicated with a combustion chamber 64 formed between the cylinder head 61 and the top of the piston 63, and is taken into the combustion chamber 64 together with the intake air A during an intake stroke in which an intake valve 66 is opened. The natural gas fuel C is ignited by electric spark of the ignition plug 62 disposed in the upper center of the combustion chamber 64. Although just an image, the combustion propagates in layers as indicated by thin lines. Then, combustion gas generated by the combustion does its work while pressing down the piston 63 during an expansion stroke involving expansion of the combustion gas. Thereafter, the combustion gas (exhaust gas) G is discharged into an exhaust system passage 67 during an exhaust stroke in which a next exhaust valve 68 is opened.
In the case of a small engine with low output or a small cylinder bore (cylinder hole) diameter or in the case of an engine in which the natural gas fuel C is readily combusted by lean combustion in a high oxygen region, the natural gas fuel C is combusted by such layered combustion so as to propagate all over the combustion chamber 64 from approximately the center of the combustion chamber 64. Thus, the force pressing down the piston 63 becomes uniform, and pressing force Fp is directed along the central axis of the piston 63 as a whole. As a result, the piston 63 is smoothly reciprocated.
However, in the case of a large engine with high output or a large cylinder bore diameter, high-temperature combustion gas G flows out toward the exhaust system passage 67 in the engine 10X. Meanwhile, since low-temperature intake air A flows into the intake system passage 65, the temperature on the exhaust system passage 67 side gets higher than that on the intake system passage 65 side. Particularly, a portion H indicated by cross-hatching in FIG. 6 is likely to be heated to a high temperature. For this reason, the natural gas fuel C is ignited by contacting with the high-temperature portion H before entering a combustion stroke, rather than being ignited by ignition of the ignition plug 62, resulting in a phenomenon called detonation (abnormal combustion) in which the combustion propagates all over the combustion chamber 64 from the high-temperature portion H side. When such a phenomenon occurs, the combustion propagates all over the combustion chamber 64 from one of the corners thereof. For this reason, the force pressing down the piston 63 becomes non-uniform, and the pressing force Fp is directed obliquely to the central axis direction of the piston 63 as a whole. As a result, the piston 63 partially hits against the cylinder 70, and can no longer be smoothly reciprocated, leading to engine trouble. The detonation is one of the causes of knocking.
Meanwhile, when the exhaust gas is subjected to aftertreatment using a three-way catalyst, in order to reduce NOx in the exhaust gas, an exhaust gas treatment device carrying the three-way catalyst is provided in an exhaust passage to reduce and remove NOx in the exhaust gas. As to the three-way catalyst, catalyst performance thereof is lowered in an air-fuel ratio lean region with much oxygen. Thus, the three-way catalyst is preferably in an air-fuel ratio rich region with low oxygen concentrations in the exhaust gas. Therefore, inside the cylinder, stoichiometric combustion (combustion with a stoichiometric ratio: complete combustion that leaves no oxygen after combustion) or rich combustion (combustion close to the stoichiometric ratio and with low oxygen concentrations) is performed. Thus, the stoichiometric combustion is performed in the cylinder by controlling the amount of intake air into the cylinder using an intake throttle valve provided in the intake passage and supplying intake air with an oxygen level required and sufficient to combust the natural gas fuel.
Since an air-fuel mixture of the intake air A and the natural gas fuel C is not readily combusted, unlike lean combustion, also in such a case of stoichiometric combustion or rich combustion, the combustion starts from the high-temperature portion H on the exhaust system passage 67 side, making the detonation likely to occur.
As described above, considering higher output and increase in size of the natural gas engine or rich combustion therein, the detonation problem needs to be resolved. It is also required to resolve a problem of heat damage to the ignition plug, in which the ignition plug is damaged by heat in an ignition plug ignition system. Meanwhile, when there is one ignition plug for spark ignition, only one ignition source makes it difficult to perform reliable ignition for each cycle, leading to a problem of poor combustion efficiency. Such poor combustion efficiency requires a large amount of fuel to obtain a desired engine output, resulting in an increase in amount of heat to be generated. As a result, various durability problems are likely to occur, such as fuel consumption, heat damage to an electrode portion of the ignition plug and heat damage to exhaust system parts.
As measures against such problems, there has been proposed a fuel control device for an internal combustion engine as described in Japanese patent application Kokai publication No. 2012-57471, for example. In the fuel control device, CNG (natural gas) fuel is injected into an intake passage by a CNG injector (CNG fuel injection device) and diesel oil is injected into a combustion chamber by a diesel oil injector (diesel fuel injection device). By mixing the CNG fuel with the diesel oil having high compression ignition properties, the CNG is combusted using the diesel oil as an ignition source. Moreover, a ratio of the CNG to the diesel oil is changed based on the maximum pressure during combustion in a combustion chamber.
As shown in FIG. 7, in a natural gas engine 10Y that simultaneously uses such diesel fuel, diesel fuel f is injected by a diesel fuel jet injector (diesel fuel injection device) 69 during a compression stroke to compress an air-fuel mixture of natural gas fuel C and intake air A. Then, as the fuel spreads inside the combustion chamber 64, the temperature of the air-fuel mixture is increased by adiabatic compression of the air-fuel mixture. When the temperature of the air-fuel mixture exceeds an ignition temperature of the diesel oil, the diesel fuel f starts to be combusted by compression ignition, and the natural gas fuel C around the ignition source is also combusted. At this combustion start point, the diesel fuel f is spreading inside the combustion chamber 64. Thus, multipoint ignition can prevent ignition from an exhaust system high-temperature portion. Accordingly, combustion is performed in the entire combustion chamber 64, and thus approximately uniform force is applied to the top of the piston 63. Therefore, pressing force Fp pressing down the piston 63 is set approximately in a central axis direction of the piston 63. Thus, the piston 63 is smoothly reciprocated. Accordingly, in the engine using both the diesel fuel and the natural gas fuel, detonation can be prevented. Also, since no ignition plug is used, no heat damage to the ignition plug occurs.
However, the natural gas engine simultaneously using the diesel fuel also has the following problem. Specifically, as shown in FIG. 8, compared with a diesel engine according to a conventional technology, which is operated with an excess air ratio λ of 2 to 8, an intake air amount is significantly reduced in a natural gas engine in which stoichiometric combustion is performed with an excess air ratio λ of 1. As a result, the compression pressure in the cylinder is lowered, and the temperature rise of the air-fuel mixture in the cylinder by the adiabatic compression is also reduced. Particularly, in the case of a low load operation region with a small engine output (horsepower), when the intake air amount is reduced to maintain the stoichiometric combustion, with reduction in fuel, the compression pressure in the cylinder is significantly lowered, leading to a problem of unstable combustion. Such a problem needs to be resolved.
With regard to such problems, the inventors of the present invention have proposed a diesel engine including an exhaust gas purification device for the purpose of reliably and continuously combusting PM collected by a DPF in a wide operation region of the diesel engine, as described in Japanese patent application Kokai publication No. 2002-349241. In the diesel engine, an exhaust cam is formed in two stages including an exhaust gas introduction cam, or an exhaust gas introduction valve is provided and an exhaust gas introduction mechanism is provided to introduce exhaust gas into a cylinder during an intake stroke. When an exhaust temperature range is a low temperature range, an intake shutter, an exhaust shutter and the exhaust gas introduction mechanism are operated to increase the exhaust gas temperature (in-cylinder temperature).
The diesel engine described above is an engine using only diesel oil as fuel, but is an engine capable of stably igniting the fuel in the cylinder, even when an intake air amount is reduced by an intake throttle valve in a low load operation condition of the engine, by using the exhaust gas introduction mechanism that increases the in-cylinder temperature by causing some of the exhaust gas in the exhaust passage to flow back into the cylinder during the intake stroke of the engine.